The invention concerns the transmission of energy by a mainspring to a barrel drum via an element conventionally called a bridle. In general, for manually wound movements, the mainsprings are connected to the drum via a fixed bridle. For self-winding automatic movements, the transfer of force occurs via a sliding bridle or slip-spring which enables the end of the mainspring to be temporarily disengaged from the barrel above an disengagement limit torque.
The fixed bridle makes it possible to maximise the properties of the mainspring, particularly the number of turns, but has the drawback of generating overtorque when the mainspring reaches its maximum degree of wind: the user only feels that the spring is totally wound once the torque becomes significantly greater. This overtorque may cause not only problems of fatigue (breakage) but also chronometric problems.
The sliding bridle is indispensable for self-winding movements, however it is difficult for the user who manually winds the movement to know, without a power reserve indicator, when the barrel is fully wound. The presence of a sliding bridle or slip-spring generates a variation in torque when the barrel reaches the maximum degree of wind between the maximum moment of slippage (MGS) and the minimum moment of slippage (MGI). Further, this system does not allow proper control of the torque value above which disengagement occurs. Finally, these components, and particularly the barrel drum, are often subject to wear during aging tests.
U.S. Pat. No. 1,495,348A in the name of Daniel ODOM discloses a barrel with a drum whose inner periphery is corrugated in a regular manner, and wherein the sliding bridle of the mainspring can cooperate with the corrugation by resilient engagement of its distal end which includes a folded portion of the spring and a bent transverse rib, whose profile is locally substantially complementary to a groove of the corrugation.
U.S. Pat. No. 1,720,289A in the name of Daniel ODOM discloses a similar drum, cooperating with a spring that includes a large outwardly projecting folded portion, at around three quarters of the outer coil. This folded portion locally forms a substantially complementary profile to a groove of the corrugation. The outer three quarters of the outer coil press on the upper portions of the corrugation when there is no excessive torque as the barrel is wound.
EP Patent Application No 2420899 in the name of ETA Manufacture Horlogère Suisse discloses a mainspring including on the inner periphery thereof friction surfaces alternated with locking structures forming projecting corners. The distal end of the spring is friction-coupled to the inner wall against which it can slip. The friction surfaces are limited, downstream with respect to the direction of slippage of the outer coil, by a rounded edge or by a reflex edge.